1. Technical Field of the Invention
The present invention relates to a developing device provided with a plurality of developing units for use in image forming apparatuses such as color copying machines, color printers and copying machines which are arranged for selectively forming monocolor images of different colors.
2. Brief Description of Related Art
Generally, a color copying machine is designed for forming colored images by using four colored developers, and a developing device used for the copying machine is provided with a plurality of developing units accommodating different colored developers. The developing units are disposed in one of the following three methods.
(1) Stationary Method: Each developing unit is disposed around a photoconductor. PA0 (2) Rotary Method: Each developing unit is disposed on a rotary support member, and a desired developing unit is positioned at a developing location opposite to a photoconductor by rotating the rotary support member. PA0 (3) Reciprocative Movement Method: Each developing unit is disposed on a reciprocative movement support member, and a desired developing unit is positioned at a developing location opposite to a photoconductor by reciprocatively moving the reciprocative movement support member.
The stationary method has a problem that the diameter of photoconductor becomes large since each one of the developing units has to be positioned opposite to the surface of the photoconductor coincidentally at equal spaces. Moreover, there occurs attenuation variation of electrostatic latent images with the passage of time because of the difference in distance between a developing location and the position where each colored electrostatic latent image is formed, which has to be rectified.
The rotary method, which is one of the moving methods as against the stationary method, is able to solve the above-mentioned problems inherent in the stationary method. However, there are problems that toner supply can not be performed efficiently since developer tends to easily fall out of a developing unit since the rotary support member is rotated and that toner is supplied only to a developing unit positioned at a predetermined rotational position. Moreover, it is difficult to always rotate a toner stirring means in the developing unit.
The reciprocative movement method needs to provide larger space than that of the rotary method in the direction of reciprocating movement, however, it can solve the problems inherent in the rotary method. Especially, it is designed for stirring and supplying toners to each developing unit even when developing operation is not being performed. It is, therefore, very advantageous for use in forming colored images where more toners are consumed than the case when black and white images are formed.
Incidentally, in a color image forming apparatus, images which occupy much dark color portions are formed and consumes large amount of toner. Accordingly, if the toner is not sufficiently stirred, the density of toner may be lowered partially. Moreover, if the toner is not stirred sufficiently, the amount of electrification of toner is lowered thereby inviting fogging on images and toner scattering since the toner supply is inevitably increased. In order to deal with the problem, it is arranged to continuously perform toner stirring operation by a stirring means for the developing units which are not under developing process by keeping them under an operating state.
For instance, Japanese Published Patent Application TOKKAI SHO No. 61-217072 discloses a device which is designed for driving a developer stirring means when a rotary support member is rotated with a developing roll rotatively driven at a low speed at the same time. However, if a developing unit is always kept under an operating state, toner is scattered from the part of developing roll and stains the inside of a machine and the leading end of a developing unit opposite to a photoconductor since the developing unit is not faced to the photoconductor. Moreover, the toner adhered to the leading end of the developing unit is attracted by electric potential on the side of the photoconductor when the developing unit approaches the position adjacent to the photoconductor thereby causing stained images. In order to avoid such troubles, it may be considered to operate the stirring means only by stopping the action of the developing roller when developing process is not performed. However, it necessitates to provide a clutch and a special actuator with its control means for switching the clutch, which inevitably invites a rise in manufacturing cost since the color image forming apparatus is provided with a number of developing units. The structure disclosed in the above patent application is limited to a rotary method only, and sufficient stirring effect can not be obtained since toner is stirred only when a developing unit is moved.